Technical Field
The present disclosure relates to methods for controlling an inverter. More specifically, the present disclosure relates to methods for controlling an inverter to interrupt an input power and speed up discharge of residual energy at a smoothing stage when the inverter is stopped for an emergency.
Description of the Related Art
An inverter is used for driving an induction motor by receiving DC power to convert it to AC power with a switching element and supplying the AC power to the induction motor. The inverter generates pulse width modulation (PWM) signal and controls the switching element with the PWM signal to generate AC power. Then, the inverter controls the pulse width of the PWM signal to change the voltage and frequency of AC power and change the torque and rotation speed of the induction motor as desired.
If a safety signal is input to an inverter while it is driven, the input power to the inverter has to be interrupted for safety. However, the inverter stands by in a safety trip mode until an operator interrupts the input power, and thus there is a risk of a secondary accident. Moreover, even after the operator has interrupted the input power to the inverter, residual energy remains in a smoothing stage, and thus there is a time delay unit the service is provided.
FIG. 1 is a graph showing voltage profile of a smoothing stage as the input power to the inverter is turned on/off when a safety signal is input.
Referring to FIG. 1, when voltage is applied to an inverter, the voltage is gradually charged at a smoothing stage during an initial charging circuit operation, and an SMPS begins to operate ({circle around (1)}). Then, the smoothing stage is continuously charged, and an initial charge relay operates such that the smoothing stage is charged up to the input power ({circle around (2)}).
Then, if a safety signal is input to an I/O unit ({circle around (3)}), the inverter enters a safety trip mode and stands by. Then, when an operator interrupts the input power ({circle around (4)}), the energy remaining in the smoothing stage is gradually discharged.
Then, when the energy remaining in the smoothing stage is discharged such that the voltage of the smoothing stage becomes a predetermined voltage ({circle around (5)}), the inverter triggers a low voltage trip. In the low voltage trip mode, the inverter stops operating and the initial charge relay does not operate, and thus the discharging of the smoothing stage becomes slower. Then, the residual energy remaining at the smoothing stage continues to be discharged and the SMPS is turned off ({circle around (6)}), such that the discharging of the smoothing stage becomes even slower.
As such, it takes longer time to discharge the residual energy at the smoothing stage, and thus resuming of the service is delayed.
There has been proposed a method for discharging the residual energy at the smoothing stage by using a discharge resistor when the inverter is stopped for an emergency. However, the discharge resistor occupies large volume and there is a risk of electric shock when attaching the discharge resistor to the smoothing stage.